how can we limit attachment sizes?

We have a limit of about 25MB file size for outbound files going through our email server. We have an issue with suppliers not receiving purchase orders because the requisition writers added very large attachments onto their reqs so the total size of the PO plus attachments goes over our email server limit. How can we limit the file sizes for attachments on our PO's? And how can we query the file sizes of added attachments?